When an aircraft detects the presence of facilities on the ground or at sea, it is sometimes desired to identify these facilities in a very short time, of the order for example of a second. This identification may be made easier when said facilities are provided with a transponder, which may be summoned by a radioelectric message emitted from the aircraft. To avoid extra onboard hardware, the responder situated in the aircraft may be used to transmit an interrogation message to the facilities detected on the ground.
By way of reminder, an airborne responder is a device making it possible to identify the aircraft on which it is placed. When an interrogation message in the standard format is received by the responder, the latter responds by transmitting a code specific to the aircraft. More particularly, “friend or foe” responders, often designated by the acronym IFF standing for “Identification Friend or Foe”, are suitable for responding, when a friendly interrogation message is received, with an encrypted code indecipherable to the enemy troops. Furthermore, the mode of operation of a responder may be reversed, the responder transmitting a message without having been previously summoned through an interrogation. In this way, an aircraft can, for example, spontaneously signal its presence to facilities situated on the ground or at sea. In the case of military use, an IFF responder operating in reverse mode can, for example, transmit interrogation messages before triggering an air-ground attack, for the purposes of ensuring that no friendly troop is situated in the targeted zone. The aircraft equipped with the responder then watches out for a possible response from a friendly facility on the ground.
Conventionally, an aircraft fitted with a responder comprises two transmission antennas linked to this responder, a first, so-called top, antenna being placed above the fuselage, a second, so-called bottom, antenna being placed under the fuselage. Indeed, the presence of two antennas is notably justified by the fact that when, for example, the aircraft banks in a turn, one of the two antennas may be masked in relation to the facilities with which it might communicate, the other antenna then making it possible to establish a communication linkup. Generally, the responder operating in reverse mode transmits a series of messages, the messages being transmitted alternately on the top antenna and on the bottom antenna.
However, this mode of operation gives rise to numerous losses of interrogation messages. Indeed, when one of the two antennas is masked, half the messages of the series are lost, that is to say not received by the ground. This loss is unfavorable to a possible air-ground identification, since it reduces the probability of a ground facility receiving a message.
A technique for solving this problem is to increase the number of interrogation messages transmitted, stated otherwise to lengthen in time the series of messages transmitted, the density of the messages over time being standardized and non-modifiable. However, this solution leads, on the one hand, to a lack of discretion and, on the other hand, to the exceeding of the acceptable duration for performing a possible identification.